The invention relates to a circuit configuration for controlling operating points of a power amplifier, in particular such as those used for mobile telephones.
In many applications, linear power amplifiers are operated with a very low quiescent current in order to achieve high efficiency levels, in particular with reduced output power levels. The electrical characteristics of such an amplifier used in the Class AB mode are very heavily dependent on the value of the quiescent current. Within the overall temperature range in which the power amplifier is operated, a constant quiescent current is a precondition for constant and reproducible electrical characteristic data.
If the supply voltages are low, quiescent current control is additionally complicated by the fact that the available voltage range of the amplifier is tightly limited. Exact quiescent current adjustment is problematic, especially in the case of power amplifiers using heterobipolar transistors with high base-emitter forward voltages of, typically, 1.3 V and a supply voltage of 3 V.
Circuit configurations known in the prior art for adjusting the operating point for a power amplifier have the problem that the maximum available voltage range is not sufficient to ensure stable adjustment of the operating point.
If the operating point of the power amplifier is configured for a low quiescent current, then, the efficiency of the power amplifier rises and thus the operating duration of an appliance with a limited energy reservoir increases. On the other hand, the maximum amplification power is limited if the quiescent current is low. In many applications, in particular including mobile radio technology, the amplification power requirements vary, however, so that it is difficult to find an optimum operating point for the amplifier if it is also necessary for the power amplifier to have as high an efficiency level as possible at the same time.
It is accordingly an object of the invention to provide a circuit configuration for controlling the operating point of a power amplifier that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which the quiescent current is as constant as possible irrespective of the temperature, thus ensuring an operating point which is as stable as possible even when the quiescent current is low. A further object of the invention is to control the operating point such that the efficiency of the power amplifier is as high as possible even if the amplification power levels vary.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for active control of an operating point of a bipolar transistor in a power amplifier circuit. The circuit configuration contains a current mirror circuit having a first bipolar transistor and a second bipolar transistor. The first bipolar transistor has a control input, a collector and an emitter, and the second bipolar transistor has a control input, a collector and an emitter. A third transistor is connected as a common emitter amplifier circuit and has a control input, a collector and an emitter. The control input of the third transistor is connected to the collector of the second bipolar transistor. A fourth transistor is connected as a common emitter amplifier circuit and has a control input, a collector and an emitter. The control input of the fourth transistor is connected to the collector of the third transistor. A fifth transistor is connected as a common collector circuit and has a control input, a collector and an emitter. The control input of the fifth transistor is connected to the collector of the fourth transistor and the emitter of the fifth transistor is connected to the control input of the second bipolar transistor. A resistor is connected to the collector of the second bipolar transistor, and a voltage source is connected to the resistor and through the resistor the voltage source is connected to the collector of the second bipolar transistor.
The term current mirror circuit should be understood in a wide sense here. The current mirror circuit is formed by two transistors whose control inputs are connected to one another, so that a collector current in one transistor produces a mirrored collector current in the other transistor. Mirrored in this context results in that both the currents behave linearly with respect to one another in the area of interest, in an ideal situation. In this case, with regard to process technology, the two transistors are preferably physically identical, so that the linearity of the two currents with respect to one another is maintained even, for example, in the event of temperature fluctuations. If the two transistors also have the same emitter-base area and if the emitters of the two transistors are at the same potential, then the ratio of the two currents is essentially unity.
The circuit configuration according to the invention allows the current through the collector of the second bipolar transistor to be regulated at a fixed value even in the event of temperature fluctuations. The quiescent current thus also remains constant in the first bipolar transistor, so that the operating point of the bipolar transistor connected in the amplifier circuit is more stable. Control of the current through the collector of the second bipolar transistor is ensured by the active feedback circuit from the collector of the second bipolar transistor to the control input of the second bipolar transistor. The common collector circuit produced with the fifth transistor results in that the output of the feedback circuit also has a low output impedance, so that high output power levels can also be accepted on the first bipolar transistor. Furthermore, the circuit according to the invention has the advantage that the possible voltage range on the first bipolar transistor is wide, since only one diode voltage need be subtracted from the total available voltage.
In one preferred embodiment, the control input of the first bipolar transistor and the control input of the second bipolar transistor are connected to one another via a resistor. The resistor allows the amplifier linearity of the first bipolar transistor to be optimized.
In a further preferred embodiment, the collector of the fifth transistor, and the collector of the fourth transistor are connected to the voltage source via a resistor, and/or the collector of the third transistor is connected to the voltage source via a resistor. This embodiment allows the circuit to be configured with a minimum number of voltage sources. Alternatively, it is possible to provide a separate voltage source with a very high current rating for the collector of the fifth transistor, in order that the fifth transistor can supply a current which is as high as possible to the control input of the first bipolar transistor when the amplifier power level is high.
The transistors which have been mentioned are preferably npn bipolar transistors, and are preferably heterobipolar transistors owing to their superiority in terms of noise response and linearity when the signals to be amplified are analog signals at high frequencies.
In a further preferred embodiment, the control input of the third transistor is connected to the reference ground potential via a resistor, and the control input of the third transistor is connected via a resistor to the collector of the second bipolar transistor, and, furthermore, the control input of the third transistor is preferably connected to the collector of the third transistor via a resistor. The choice of these three resistors allows the operating points of the common collector amplifiers to be set, and thus allows the gain and linearity to be adjusted. Further resistors can also be fit.
The first and second bipolar transistors are advantageously constructed and disposed with respect to one another in such a manner that they have the same thermal characteristics and are largely subject to the same temperature. For a current mirror circuit, it is important for the two bipolar transistors also to behave in the same way in response to thermal changes since, otherwise, the behavior of the current mirror circuit as the current source is adversely affected. It is thus advantageous for the transistors that are part of the current mirror circuit, in particular the first bipolar transistor and the second bipolar transistor to be placed physically as close to one another as possible and, as far as possible, to be produced in the same production process with the same parameters. These two characteristics are best ensured if the two bipolar transistors are produced on the same integrated circuit in the same procedure and are placed as close to one another as possible. The best thermal connection between the two bipolar transistors is achieved, however, if the two bipolar transistors are part of a single bipolar transistor with separate collectors.
Since the circuit according to the invention has bipolar transistors of only one polarity, the circuit can also be produced completely in heterobipolar transistor production processes. The circuit is preferably produced in a GaAs or InGaP process step, so that the transistors used in the circuit configuration according to the invention are heterobipolar transistors. However, the circuit can also be produced, for example, using the SiGe production process or other heterobipolar transistor processes.
In a further preferred embodiment, capacitors are also fit in order to reduce interference signals into the circuit configuration. In particular, capacitors can be fit between each of the control inputs of the second through fifth transistors and the reference-ground potential.
In a further preferred embodiment, a resistor is in each case connected between the reference-ground potential and the emitters of the third and/or fourth transistors. These resistors allow, for example, the linearity of the common emitter amplifier to be adjusted.
The first bipolar transistor is part of one or more amplifier stages in a power amplifier. The amplifier stage is preferably an amplifier in a common emitter circuit having a supply voltage that is connected via a resistor and/or an inductance to the collector of the first bipolar transistor. The emitter of the first bipolar transistor is in this case connected to the reference ground potential, preferably also via a resistor. The signal to be amplified is preferably passed via a coupling capacitor to the control input of the first bipolar transistor.
According to the invention, a circuit configuration for controlling at least two operating points of a bipolar transistor in a power amplifier circuit is furthermore provided. The circuit configuration contains a first current mirror circuit having the first and a second bipolar transistor and is active in a first circuit state, so that a first current through the collector of the second bipolar transistor produces a first current mirror through the collector of the first bipolar transistor. A voltage at the collector of the second bipolar transistor is fed back via an active feedback circuit to the control input of the second bipolar transistor.
A second current mirror circuit is provided and has the first and a third bipolar transistor and is active in a second circuit state so that a second current through the collector of the third bipolar transistor produces a second current mirror through the collector of the bipolar transistor. A voltage at the collector of the third transistor is fed back through a second feedback circuit to the control input of the third bipolar transistor. A switch circuit for switching between the first and the second circuit state is provided.
In the configuration according to the invention, the two operating points of the first bipolar transistor are each governed by one of the two current mirror circuits. The relevant current mirror circuit is in this case the active current mirror circuit. The circuit configuration according to the invention thus makes it possible to use only one switch to operate the first bipolar transistor, by switching to at least two different operating points.
The position of the operating points in the family of characteristics of the first bipolar transistor can in this case be adjusted essentially by the resistances of the resistors and the transistor parameters. Therefore, the circuit according to the invention can be constructed such that the quiescent current and quiescent voltage for a power amplification application with a variable output power level can be switched to optimum values with regard to the power consumption. The power efficiency can thus be increased significantly, even for linear amplification.
In one preferred embodiment, the collector of the second bipolar transistor is electrically connected to a voltage source via a resistor. The first current through the collector of the second bipolar transistor, and thus through the first bipolar transistor, is thus defined in the first circuit state.
In one preferred embodiment, the second feedback circuit contains only a line with or without a non-reactive resistor. Furthermore, the collector of the third bipolar transistor is preferably connected to a voltage source via a resistor. The second current through the collector of the third bipolar transistor is essentially defined in this way. The current through the first bipolar transistor when small AC signals are applied to the input of the power amplifier is thus also defined in the second circuit state.
In one preferred embodiment, the active feedback circuit has a voltage amplifier. The voltage amplifier results in that any voltage change (caused by a change in the first current) at the collector of the second bipolar transistor is amplified and is fed back to the control line of the second bipolar transistor, thus counteracting the change in the first current. Therefore, the first current is also stabilized in the event of temperature changes. The active feedback circuit preferably has two series-connected common emitter amplifiers since these amplify an input voltage in a simple manner and, when connected in series, maintain the polarity of an input signal.
The active feedback circuit preferably has a low-impedance output stage, so that the control input of the first bipolar transistor can be driven with a low impedance even when the output power levels are high.
The output stage of the active feedback circuit preferably has a high impedance below a minimum threshold voltage difference between the collector voltage and the control input voltage of the second bipolar transistor. This characteristic allows the active feedback circuit to be deactivated in a simple manner.
Deactivation of the active feedback loop is necessary, for example, if one wishes to deactivate the first current circuit in order to drive the first bipolar transistor using the second current mirror circuit. The output stage of the active feedback circuit thus preferably has a common collector circuit which, in linear operation, has a low-impedance output, but has a very high-impedance output when the voltage between the emitter and the control input is less than the diode voltage of the transistor.
The first current mirror circuit is preferably configured as the circuit configuration previously mentioned. Such a circuit provides the already described advantages of temperature-stabilized quiescent-current operation in the first bipolar transistor, and represents a low-impedance voltage supply for the control input of the first bipolar transistor which is to be supplied by it. Furthermore, this type of active feedback provides a way for deactivating the active feedback circuit by the voltage between the control input and the collector of the second bipolar transistor being set, switched from the exterior, to be less than the threshold voltage difference.
The control input of the third bipolar transistor is preferably connected via a resistor to the first bipolar transistor and to the second bipolar transistor, with the resistor having a considerably greater resistance than the low output impedance of the active feedback circuit in the non-high-impedance state. In the second circuit state, the resistor essentially governs the output impedance with which the DC control input of the first bipolar transistor is supplied with voltage. The resistor preferably has a considerably greater resistance than the output impedance of the activated active feedback circuit, and a considerably lower resistance than the output impedance of the deactivated active feedback circuit.
The switch circuit is preferably of such a type that, in the second circuit state, it sets the collector voltage of the second bipolar transistor to a threshold voltage value which is less than the collector voltage in the first circuit state. The threshold voltage value is preferably so low that, in the second circuit state, the voltage between the collector and control line of the bipolar transistor is less than the threshold voltage difference. Such a switch circuit allows the active feedback circuit to be deactivated, so that the quiescent current in the first bipolar transistor is controlled by the second current mirror circuit.
The switch circuit preferably has a transistor disposed in parallel with the second bipolar transistor, with the collector of the transistor being connected, preferably via a resistor, to the collector of the second bipolar transistor, and with the transistor being switched off in the first circuit state and being switched on in the second circuit state by applying a control voltage to the control input. The resistor in this case allows the threshold voltage value to be adjusted.
The DC control input of the first bipolar transistor, which has a comparatively high impedance in the second circuit state, furthermore results in that it is possible to operate with a particularly high power efficiency in a third mode. The high-impedance DC control input results in that it is possible, when the AC input signals are high at the same time, for a current to be drawn at the control input of the first bipolar transistor that is so high that the control input of the first bipolar transistor falls to a lower value than when the AC signals are small. The drop in the control input voltage to the first bipolar transistor results in that the active feedback loop becomes active once again, so that the low voltage at the first bipolar transistor is supplied with a low impedance. The first bipolar transistor can thus produce very powerful signals, even if they are also non-linear. This type of amplification is used in particular for the mobile radio transmitters in the AMPS standard.
The control input of the second bipolar transistor is preferably connected via a resistor both to the control input of the first bipolar transistor and to the control input of the third bipolar transistor. The resistor, whose resistance is preferably the same as that of, is multiplied by the ratio of the emitter-base areas of the bipolar transistor and the bipolar transistor. It is preferably used to ensure that the voltage drop (produced by the base current) across the resistor is approximately of the same magnitude as the voltage drop (produced by the base current) across the resistor. This measure improves, inter alia, the linearity of the channel currents in the first and second bipolar transistors and when the first current mirror circuit is active.
The control input of the first bipolar transistor is preferably connected via a resistor both to the control input of the second bipolar transistor and to the control input of the third bipolar transistor, with the resistance of the resistor being less, preferably more than four times less, than the resistance of the resistor. The resistor allows the linearity of the power amplification of the bipolar transistor to be optimized for AC input signals.
The first bipolar transistor is preferably part of a power amplifier stage having an amplifier input that is connected via a coupling capacitance to the control input of the first bipolar transistor. This allows AC input signals and, in particular, radio-frequency AC signals to be amplified linearly with regard to voltage and current. The maximum output power of the first bipolar transistor is in this case governed essentially by the characteristic data of the first bipolar transistor and by the maximum current available in the control input. The amplifier stage is in this case preferably a common emitter amplifier.
The first, the second and the third bipolar transistor are preferably constructed and disposed with respect to one another in such a manner that they have the same thermal characteristics. This measure results in that the transistors which are relevant for the current mirror have largely identical characteristics at the relevant temperatures, except for an area factor that is governed-by the base-emitter areas. In particular, it is advantageous if, for this purpose, the first, the second and the third bipolar transistor are disposed in the immediate vicinity of one another in an integrated circuit.
Furthermore, the first, second and third bipolar transistors are heterobipolar transistors, in particular from a GaAs, InGaP or SiGe heterobipolar transistor process. Heterobipolar transistors allow very fast, linear, low-noise amplifier circuits to be constructed as are required in particular for mobile telephone technology. Since the power amplifier is preferably equipped with a heterobipolar transistor, it is advantageous for the operation of the current mirror circuits for the corresponding current mirror circuit transistors to be heterobipolar transistors as well.
Furthermore, the circuit configuration according to the invention is preferably used for mobile telephones, where it is used in particular for amplification of the signals to be transmitted, where the aim is to amplify signals in the Gigahertz frequency range to power levels of more than 30 dBm with high power efficiency.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for controlling the operating point of a power amplifier, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.